This invention relates to a process for producing microcapsules, en masse, in an aqueous manufacturing vehicle and to the capsules produced thereby. More particularly, this invention relates to microcapsules containing a substantially water-insoluble additive in the form of finely divided solid or liquid material incorporated at the surface of the microcapsule wall under a very thin polymer film. Microcapsules are capsules of a diameter of about 5-5000 microns. Examples of such substantially water-insoluble additives include pearlescent materials, metal flakes, optical brighteners and solid or dissolved ultraviolet absorbers.
Microcapsules are known, in which additives such as pearlescent agents or carbon black are distributed throughout the capsule wall. Thus U.S. Pat. No. 4,115,315 and patents cited therein teach processes whereby opaque material is dispersed throughout the wall material. In terms of providing an opaquing, or, effect, this procedure is effective. However, if a highly reflective or absorptive surface is desired, deposition of the additive at the surface of the capsule is clearly more effective. The referenced procedures, under manufacturing conditions, have also been found objectionable because some additive tends to find its way into the core material. This is avoided by the present method, in which the additive is applied only after the wall has been deposited in a first microcapsule coating.
It is thus an object of the invention to provide microcapsules having an additive disposed at the surface of the capsule, covered only by a thin film, rather than distributed throughout the capsule wall.
It is a further object of the invention to provide microcapsules in which the core is not adulterated by admixture with the additive.
It is an additional object of the invention to provide a process requiring a shorter reaction time than the processes of the prior art referred to above.
It is also an object of the invention to provide microcapsules in which smaller quantities of additive are needed because of optimal distribution at the surface of the capsule.
These and other objects and advantages provided by this invention will become apparent from a consideration of the following disclosure.
A variety of finely divided powder or liquid additives can be disposed at the outside of microcapsules by the method of the invention, including pearlescent materials, metal flakes, optical brighteners and ultraviolet absorbers. Substantially, water-insoluble solutions of such additives can be employed.
Pearlescent particles especially preferred are typically flat mica carriers or like silicas. In a preferred embodiment of the invention these mica carrier materials are coated with a titanium dioxide pigment. The particles, in the form of platelets, generally have a length of about 5-35 microns along their longest dimension. The amount of titanium dioxide coated on the mica is typically in a range of about 15-50% of the total weight of the particles. A convenient commercially available material is marketed as Satina 100 of Mearl Corporation. Suitable metal flakes are typically finely ground, flattened metals in micron-size particles, the surfaces of which are highly reflective. Especially suitable are such metals as aluminum and nickel, but iron, cobalt and other metals can be employed, depending on the demands of the user, which may depend on electrical, magnetic, incendiary, chromophoric, and other properties of the metal used.
Optical brighteners which can serve as additives for laundry products are materials which, when impinged by ultraviolet radiation, enhances the light emitted in the visible spectum. Typically suitable examples include disodium 4,4.sup.1 -bis (4,6 dianilino-s-triazin-2-ylamino)-2,2.sup.1 -stilbenedisulfonate, known commercially as Arctic White, and 2-hexylamino 1,9-methylpyridinoanthrone (Fluorescent Yellow C-4) and 2-alkyl homologs thereof.
Ultraviolet absorbers suitable for the purposes of this invention are compositions which protect a substrate from potentially harmful utraviolet radiation including carbon black and 5-chlorobenzotriazoles additionally substituted in the 2-position by phenolic groups such as 2-(5-chloro-2H-benzotriazol-2-yl)-6-1,1.sup.1 -dimethylethyl)-4-methylphenol known commercially as Tinuvin 326. Inclusion of an ultraviolet absorber affords protection to agricultural agents which are susceptible to degradation by ultraviolet radiation, as is observed in the case of polyhydrosis virus.
Preferred embodiments of this invention are microcapsules containing as the core material such oily materials as mineral oils, vegetable oils, animal oils, oils prepared by modification of natural oils and oils of purely synthetic origin as halogenated hydrocarbons. Specific examples are white mineral oil such as the product known commercially as Blandol, paraffin oil, cotton seed oil, soybean oil, corn oil, olive oil, castor oil, safflower oil and other fruit skin oils. Representative animal oils are fish oils and lard oil.
The use of cosmetic grade white mineral oil cores is especially preferred for use in microcapsules with pearlescent additives because these products can be added to such cosmetic products as hair conditioners in shampoos. Thus, addition of 0.1-0.4% by weight of the hair preparation provides a formulation useful for dispersing mineral oil into hair on use by rupture of the capsules. The pearlescence in the capsules is visible throughout the liquid hair preparation and produces an aesthetically desirable appearance.
The core material may also be a water-insoluble substance such as a chemical or biological pesticide, a fluorescent or phosphorescent agent.
While the overall sequence of the instant process is new, certain individual steps described in U.S. Pat. No. 4,115,315, issued Sept. 19, 1978 and the prior art cited therein are applicable to the steps of first forming the capsule prior to deposition of the additive, and of hardening the capsule. According to the preferred method of this invention, a primary capsule, having as at least one wall material component an anionic coacervation phase hydrophilic polymeric colloid, is prepared by a conventional separation process. Thus, deposition of colloid around the nuclei of water-insoluble core material can be produced by coacervation and/or phase separation which can be brought about by adjustment of the acidity of a mixture of at least two different colloid polymeric sols in which the core particles or droplets are dispersed. The two kinds of colloids must have different electric charges in the mixture prior to coacervation in order to permit coacervation to occur. As is recognized in the art, one can use salt or polymer-polymer incompatibility for this preliminary step. Hydrophilic colloidal materials suitable include gelatin, albumin, alginates such as sodium alginate, casein, agar-agar, starch, pectins, Irish moss and gum arabic.
Carboxymethylcellulose is a particularly useful negatively charged polymer which forms an excellent liquid polymer coacervate with positively charged gelatin. Other negatively charged polymers, such as gum arabic, carageenan sodium hexametaphosphate, polyvinyl methyl ether, maleic anhydride copolymers such as ethylene maleic anhydride copolymer and polyvinyl methyl ether maleic anhydride copolymer can be used in lieu of carboxylmethylcellulose. However, carboxymethylcellulose is especially desirable for use in the subsequently described process of Example 1, because it is compatible with the post-treatment step using ureaformaldehyde. By way of contrast, substitution of a gelatin-gum arabic capsule requires an intermediate washing or chemical treatment to cause the capsules to accept ureaformaldehyde deposition efficiently.
In a preferred embodiment of this invention, the initial formation of a first or primary capsule is carried out by a conventional coacervation/phase separation technique. As in the usual capsule formation, mentioned above, a colloid is deposited around the nuclei of core material by coacervation/phase separation using positively and negatively charged polymers and adjustment of acidity. It should be noted that microencapsulation is promoted by cooling the batch to 30.degree. and on further cooling to about 20.degree., solidification satisfactory for the subsequent steps of the inventive method is achieved. In normal capsule manufacture, capsules are typically chilled to about 10.degree. to harden the capsules, as the capsules are cross-linked in the gel state. However, for the purposes of this invention it is sufficient to cause a physical setting of the wall material so as to permit an efficient separation as, for instance, decantation. The stirring is halted when the microcapsule wall has solidified, water is advantageously added and the microcapsules can be separated by decantation. By this decantation, extraneous or undeposited coating material is removed, which would otherwise consume some of the additive to be deposited in the subsequent deposition step, leading to inconstant and non-reproducible results.
In that deposition step, the capsules are first stirred in water, after which the desired additive is added with stirring to form a fine dispersion. The batch is agitated, preferably at a temperature of about 25.degree.-35.degree., at which a cationic hydrophilic colloid, such as a gelatin solution is added in a small quantity but sufficient to envelope the additive and subsequently deposit it at the surface of the capsule wall under a thin polymer coating as a result of a second chemisorption reaction between the oppositely charged polymer of the film former and the polymer in the wall. Temperatures above 35.degree. are undesirable because the primary capsule walls set arond the core tend to be weakened. On the other hand, if the temperature of the batch is too low, local precipitation of the hydrophilic film forming colloid (such as gelatin) would occur. Where the cationic hydrocolloid used is gelatin, a pH of 3.7 to 4.2 is preferred. Stirring is continued for a few minutes to assure deposition of the additive.
The achievement of deposition of a large amount of additive on the surface of the capsule, using only a small amount of film forming hydrophilic colloid such as gelatin, was unexpected. From the teaching of the prior art, it was believed that an additive such as mica could be deposited efficiently only after thorough washing of the capsules and microencapsulation by a conventional second encapsulation process, using both an anionic and cationic hydrocolloid polymer.
In the subsequent hardening of the capsule, one may employ chilling, but it is more desirable to use a conventional chemical reaction or complexing process using known hardening agents for organic hydrophilic polymers. Suitable hardening agents include glutaraldehyde, formaldehyde, glyoxal, cinnamaldehyde, tannic acid and compounds producing a similar effect on the organic polymer in aqueous media.
After cross-linking with an agent such as glutaraldehyde, the capsules can advantageously be subjected to a plastic treatment by grafting of unreformaldehyde, resorchinol-formaldehyde or other polymers to a gelatin-base or equivalent capsule wall. Advantages in use of the exemplified gelatin-carboxymethylcellulose system in carrying out this post-treatment grafting step are mentioned herein above.